I am beginning to understand how to read headphone measurements to the point where I can look at them and see whether a headphone is warm, bright, etc. Basically if it's worth checking out.

On the other hand, when I look at amp measurements like these, I have a hard time understanding how one amp is commonly accepted as warm (eg Violectric V200) or another one as bright (eg Burson Conductor) when all the amps on the page seem to have essentially flat frequency response other than the extremes of human hearing (and less than 1db roll off even at those extremes).

Is there a way of reading amp measurements to help decide whether an amp's signature may suit one's tastes? And is anyone doing such measurements for different tubes?

Sadly, no, IMO. To my eyes---and I have to say I'm not really qualified to interpret amp measurements well---you can only tell if the amp is poorly designed or is doing something weird. You can tell a lot about how much power supply noise something has, but usually it's low enough that it really doesn't effect the listening experience much. Crosstalk measurements MAY be telling when it's too high. The really tough one to interpret is the TIM/DIM measurement; still learning about it, but you have to look for the addition of some small blips in specific positions in frequency. That one MIGHT be able to reveal some harshness in the listening experience.

I guess when someone says an amp is warm sounding, they mean something different than when they say a headphone is warm sounding. Because the latter has a correlate in the frequency response measurements, and the former does not. Is that right?

I guess when someone says an amp is warm sounding, they mean something different than when they say a headphone is warm sounding. Because the latter has a correlate in the frequency response measurements, and the former does not. Is that right?

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I believe that is correct. Same goes for DACs, as I think they all measure flat, but still have sonic characteristics like "warm". I don't even pretend to understand what that means.

The harmonic distortion measurement might give some indication of warmth. Tubes tend to have strong second order distortion which is typically perceived as warm and pleasant. In the examples you linked, there is a 500 Hz fundamental and the peak at 1000 Hz is the second harmonic. The level of this 1000 Hz harmonic can give some indication of warmth.

See this example where I've measured the harmonics of a 1000 Hz signal on the Bottlehead Crack. You can see that the second harmonic is only 40 dB down from the fundamental where on most solid state amps, it will be down >100 dB.

The harmonic distortion measurement might give some indication of warmth. Tubes tend to have strong second order distortion which is typically perceived as warm and pleasant. In the examples you linked, there is a 500 Hz fundamental and the peak at 1000 Hz is the second harmonic. The level of this 1000 Hz harmonic can give some indication of warmth.

See this example where I've measured the harmonics of a 1000 Hz signal on the Bottlehead Crack. You can see that the second harmonic is only 40 dB down from the fundamental where on most solid state amps, it will be down >100 dB.

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Thanks, that is very interesting. Is there a measurement correlate to when someone says an amp is smoothed over, veiled, or lacks detail? It seems like there should be a way to demonstrate such issues.

There are many key aspects to amp quality that are either not commonly reported on (ie, slew rate) or are not measurable. From basic stats, shoot for SNR above 110, and THD below 0.03 (with the understanding tube amps will be skewed to the worse).

A quick and dirty way to determine the overall quality of an amp is to do a google image search for the model in question. The internals will give you clues. For example, here is an amp that LOOKS good:

A quick and dirty way to determine the overall quality of an amp is to do a google image search for the model in question. The internals will give you clues. For example, here is an amp that LOOKS good:

Ok I'll grudgingly agree that a quick'n'dirty evaluation can be done by looks (size/weight/arrangement), but the rest of your generalizations are hazy. Yes a big bank of capacitors should give you plenty of reserve, but that's no guarantee of dynamics or piercing highs. Discreet does not guarantee low noise. Multiple output devices does not guarantee slew rate or dynamics.

That second amp is also a <$100 class D amp which almost fits into a pocket. At least stay within the same topology if you're going to be making a comparison based on looks.

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Is there a way of reading amp measurements to help decide whether an amp's signature may suit one's tastes? And is anyone doing such measurements for different tubes?

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Even harmonics are predominant in tube amps and FET transistors, hence tube-sound lovers might like even harmonics for sure.

"The even harmonics provides the complex, warm, rich sound that so many guitarists desire. Add to that a valve amp's ability to produce somewhat dissonant yet driving sounds when a guitarist attacks strings harder and turns rhythm playing into lead playing, and valve function creates just the harmonics a rock guitarist needs." Source: https://phys.org/news/2017-02-physics-musicians-valve-amps.html.

"Many audio experts believe that the sound quality of a high performance FET op amp is superior to that of bipolar op amps. A possible reason for this is that bipolar designs generate greater odd-order harmonics than FETs. To the human ear, odd-order harmonics have long been identified as sounding more unpleasant than even-order harmonics. FETs, like vacuum tubes, have a square-law I-V transfer function which is more linear than the exponential transfer function of a bipolar transistor. As a direct result of this square-law characteristic, FETs produce predominantly even-order harmonics". Source: http://www.ti.com/lit/ds/symlink/opa604.pdf.

I personally found that "soft trebles" and "laid-back bass" are a direct result to a very-aggressive low & high pass filters (probably same applies to DACs, not sure), that could create phase-shift/time-delay. Especially if speakers are involved, a very small phase difference from one channel to another (a tolerance of 5% for the resistors & caps from the LP and HP filters could do that!) will move bass frequency cancellation more to the left or right of the room. In stereo systems we can't adjust phase between the channels like we adjust in subs, so...one amp might sound more bass-shy than another because of a small phase difference, even if they're both perfectly flat in synthetic measurements. Some info about phase delay here: https://en.wikipedia.org/wiki/Group_delay_and_phase_delay. Also, my above assertion seems to be backup-ed somehow in this article https://www.amplifier.cd/Tutorial/Phasenverschiebung/Phaseshift.htm:

"In order to ensure however a small phase shift of e.g. only one degree with 20 kHz, the range of the Hifi amplifier must be nevertheless already properly high, a range of 50 kHz is not sufficient by far not. A high bandwidth of a Hifi amplifier is favourable and necessary, in order to achieve a very small phase shift within the audio range. Thus linear distortions can be reduced very much".